An important issue in magnetic recording is to increase the density of recording. Current magnetic media comprise a continuous recording layer formed of a large number of particles. The magnetic information is recorded in two dimensions defined by the bit length and bit width (or track width), with one bit of information comprising multiple magnetic particles. A transition region exists between two written bits. The transition region has a finite length due notably to inter-particle dipolar coupling, the randomness of the position of the particles, and the distribution of properties within the particles. The size of this transition region limits the recording bit length. To increase storage density, the bit length and/or the bit width must be reduced. However, thermal fluctuations, signal-to-noise and the intrinsic transition region are limits to the reduction of the bit dimensions.
Three dimensional recording is a solution to further increase recording capacity. An example of multi-level recording is a medium of N different layers, each layer having a different switching field. However, N separate write steps are required to set the magnetization in the layers, because first the hardest layer is set, then the next hardest, . . . and finally the state of the lowest coercive layer is set. Another example uses assisted recording, such as thermally assisted recording, coordinating multiple disciplines but still requiring N separate write steps.